Migrate start-notebook.sh to bash

Based on

> Stop using bash, haha 👍

from jupyter#1532.

If there's more apetite for this, I'll try to migrate
`start.sh` and `start-singleuser.sh` as well - I think they should
all be merged together. We can remove the `.sh` suffixes for
accuracy, and keep symlinks in so old config still works. Since
the shebang is what is used to launch the correct interpreter,
the `.sh` doesn't matter.

Will help fix jupyter#1532,
as I believe all those things are going to be easier to do from
python than bash

images/base-notebook/start-notebook.sh

@@ -1,22 +1,42 @@
-#!/bin/bash
+#!/usr/bin/env python -u
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
+import os
+import sys
+import shlex
 
-set -e
 
-# The Jupyter command to launch
-# JupyterLab by default
-DOCKER_STACKS_JUPYTER_CMD="${DOCKER_STACKS_JUPYTER_CMD:=lab}"
+# If we are in a JupyterHub, we pass on to `start-singleuser.sh` instead so it does the right thing
+if "JUPYTERHUB_API_TOKEN" not in os.environ:
+    print(
+        "WARNING: using start-singleuser.sh instead of start-notebook.sh to start a server associated with JupyterHub.",
+        file=sys.stderr,
+    )
+    os.execvp("/usr/local/bin/start-singleuser.sh", sys.argv[1:])
 
-if [[ -n "${JUPYTERHUB_API_TOKEN}" ]]; then
-    echo "WARNING: using start-singleuser.sh instead of start-notebook.sh to start a server associated with JupyterHub."
-    exec /usr/local/bin/start-singleuser.sh "$@"
-fi
 
-wrapper=""
-if [[ "${RESTARTABLE}" == "yes" ]]; then
-    wrapper="run-one-constantly"
-fi
+# Wrap everything in start.sh, no matter what
+command = ["/usr/local/bin/start.sh"]
 
-# shellcheck disable=SC1091,SC2086
-exec /usr/local/bin/start.sh ${wrapper} jupyter ${DOCKER_STACKS_JUPYTER_CMD} ${NOTEBOOK_ARGS} "$@"
+# If we want to survive restarts, tell that to start.sh
+if os.environ.get("RESTARTABLE") == "yes":
+    command.append("run-one-constantly")
+
+# We always launch a jupyter subcommand from this script
+command.append("jupyter")
+
+# Launch the configured subcommand. Note that this should be a single string, so we don't split it
+# We default to lab
+jupyter_command = os.environ.get("DOCKER_STACKS_JUPYTER_CMD", "lab")
+command.append(jupyter_command)
+
+# Append any optional NOTEBOOK_ARGS we were passed in. This is supposed to be multiple args passed
+# on to the notebook command, so we split it correctly with shlex
+command += (
+    []
+    if "NOTEBOOK_ARGS" not in os.environ
+    else shlex.split(os.environ["NOTEBOOK_ARGS"])
+)
+
+# Execute the command!
+os.execvp(command[0], command)